In the manufacture of integrated circuits (IC), patterns representing different layers of the IC are fabricated using a series of reusable photomasks (“masks”) to transfer the design of each layer of the IC onto a semiconductor substrate during the manufacturing process. These layers are built up using a sequence of processes and resulted in transistors and electrical circuits. However, as the IC sizes continue to shrink, meeting accuracy requirements as well as reliability in multiple layer fabrication has become increasingly more difficult.
Photolithography uses an imaging system that directs radiation onto the photomask and then projects a shrunken image of the photomask onto a semiconductor wafer covered with photoresist. The radiation used in the photolithography may be at any suitable wavelength, with the resolution of the system increasing with decreasing wavelength. With the shrinkage in IC size, for printing in sub-32 nm half pitch node, extreme ultraviolet (EUV) lithography with a typical value 13.5 nm is one of the leading technologies for 16 nm and smaller node device patterning. The pattern is formed from absorptive features or lines etched into the mask.
A patterning issue in EUV lithography is the shadowing effect due to oblique illumination. This effect can cause critical dimension (CD) errors up to a few nanometers. Therefore, this effect needs to be compensated during mask synthesis.